Global Buckling Design for Flexible Flowlines · 2019. 2. 8. · Pipe-lay Pressurization Trench Backfill Depressurise Leak Test Operate Trench and Backfill The flexible is pressurised

1Subsea Expo2019 – Mostafa Tantawi


Global Buckling Design for Flexible FlowlinesChallenges, Approach and Lessons Learned

Mostafa Tantawi


• Background to flexible lines

• Mechanical behavior of Flexible vs Steel.

• Why flexible buckling is important

• Flexible buckling in project cycle

• Design of flexibles against global buckling and mitigation techniques

• Case Studies

• Conclusions and learned lessons

Agenda


When? Just over 71 years ago – Operation Pluto saw the installation of the first flexible pipeline in support of D-day landings.

17 off 3-inch and 2-inch lines extending more than 500 miles across the English Channel.

Background to Flexible Lines


Different layers of different materials that act together and allow for unique behavior.

• Internal carcass – for prevention of collapse

• Internal liner – primarily an internal fluid barrier

• Pressure armour wires – provides pressure containment

• Tensile armour wires – sustaining tensile loads and internal

pressure

• Outer sheath – resists mechanical damage and prevent

seawater ingress.

What is a Flexible Flowline

Conventional Configuration


The mechanical behaviour of Flexible flowline is widely different than steel.

• Significant variation with pressure and temperature

• Significant variation between different layer arrangements

• Significant variation with wall status

• Low non-linear bending stiffness

• Lower axial stiffness

• High expansion under pressure and temperature

Mechanical Behaviour of Flexible Flowlines vs. SteelKey Differences


Mechanical Behaviour of Flexible Flowlines vs. SteelAxial And Bending Stiffness


Mechanical Behaviour of Flexible Flowlines vs. SteelExpansion Coefficients


On seabed flexible flowlines on the seabed generally do not buckle due to the following reasons:

• Low bending stiffness – easy to move on the seabed

• Comparably lower EAF for same size rigid line with same conditions (especially if high temperature)

However, the requirement of fishing protection in shallow waters would mean that burial/rockdump is to be placed.

Therefore, the cover would need to be designed to limit lateral/vertical movement whilst ensuring the following :

• No localisation of movement that may violate flexible minimum bending radius

• The protection specification is not violated by flexible movement.

Why Flexible Buckling is Important


Cost Benefit Analysis

• Protection Philosophy (Cost benefit Analysis)

• Protection Requirements

Predictive Global Buckling Analysis

• Minimum Depth of Cover/Rock Blanket Height

• Rock Tonnage Estimation

• Practicality of Pre-PressurisationTechnique

OOS Analysis

• Final Rockdump Schedule

• Verification of as-Built Status

Concept/Pre-FEED FEED/Detailed Design Execution

Global Buckling Design in Project Cycle


Design Considerations:

• Identify design Load Cases based on installation and commissioning sequence;

• Flexible wall status for each of the design load cases

• Flexible non-linear behaviour;

• Flexible limiting criteria (MBR/ compression limit).

Design of flexibles against global buckling


Pre-Pressurisation Technique: An approach where flexible is pressurised prior to trenching/rockdump. This results in locked

in tension which helps reducing the compressive effective axial force under design conditions.

Pre-Pressurisation Technique



Pipe-lay

Pressurization

Trench

Backfill

Depressurise

Leak Test

Operate

Trench and Backfill

The flexible is pressurised up to leak test pressure on the seabed prior to trenching and

backfilling. The following are the key considerations must be addressed during the

design stage:

• The integrity of the flowline when pressurised on Seabed.

• Pressurisation on the seabed is likely to result in tight curvatures which must be

manoeuvred by trenching tool. Therefore, tracked tools are probably more appropriate.

• The depth of the trench shall be sufficient to restrain the flowline after depressurisation

to avoid surfacing.

Trenching and Backfilling Considerations



Pipe-lay

Pressurization

Rockdump

Depressurise

Leak Test

Operate

Trench and Backfill

The flexible is pressurised up to leak test pressure on the seabed prior rockdump. The

following are key consideration must be addressed during the design stage:

• The integrity of the flowline when pressurised on Seabed.

• Pressurisation on the seabed is likely to result in tight curvatures. Rockdump shall be

sufficient to restrain the lateral movement following depressurisation.

• Locked in tension shall be carefully evaluated during predictive stage. No over estimation.

Surface Lay and Rockdump Considerations



Case Study 1Surface Lay and Rockdump

Parameter Unit Oil Production Flowline

Inner Diameter inch 7

Design Pressure Barg 180

Design Temperature oC 60

Length km 4.2

Pressurisation Pressure Barg 399

• Installed on Seabed.

• Pressurised to 400 barg.

• Pipeline significantly mobilised under pressure at multiple locations.

• Rockdump to nominal for protection.

• Depressurise under rock – flowline cannot be restrained due to soft soil.

• High potential of cyclic movement under design conditions.

• Risk on pipeline integrity (localisation of curvatures) along with severe disturbance of protection cover.

• Limitation applied on number of cycles associated with stringent inspection survey requirements

Challenges:

• Very Soft Seabed

• Very Fast Track Schedule

• Limited Weather Window


Case Study 2Trench and Backfill

Parameter Unit Oil Production Flowline

Inner Diameter inch 6

Design Pressure Barg 220

Design Temperature oC 120

Length km 5

Pressurisation Pressure Barg 242

• Installed on Seabed.

• Pressurised to 242 barg.

• Pipeline mobilised under pressure at multiple locations, mobilisation assessed and matched predictions of predictive design. No integrity or trenchability issues.

• Pipeline backfilled to an average of 1 m DOC.

• Depressurised.

• Survey showed minimal movement of pipeline following depressurising highlighting thesuccess of pre-pressurisation technique.

• The approach eliminated the requirement of any rockdump resulting ina considerable saving to the project.

Challenges:

• Fast Track Schedule

• No – As-laid data (prior to pressurisation)


Conclusions, Recommendations & Learned Lessons

• Global buckling is important to be addressed as early as concept stage to ensure robust and economical design.

• Early engagement with flexible manufacturer during tendering phase to minimise flexible axial strain (i.e. effective axial force).

• Pre-trench pressurisation is generally effective however must confirmed by predictive analysis.

• The non-linearity in flexible behaviour should be captured in the analysis. Over conservatism sometimes renders no solution.

• Understanding of flexible mechanical properties under cyclic behaviour is important in of an undesirable unrestrained scenario.

• It is beneficial to survey the flexible straight after lay (prior to pressurisation) to double check the effectiveness of pressurisation technique.

• It is imperative that the global buckling analysis (especially during the execution phase) is reflective of the offshore sequencing and operation and

analyses shall be performed at each step of analysis as follows:

• Following pipelay and prior to pressurisation. This will confirm the integrity of pipeline when pressurised and ensures no scenario of localised

imperfection.

• Following pressurisation and prior to rockdump. Identify rock requirements after rockdump to restrain the pipe.

• Following rockdump. Ensures that design rock requirements are met and the pipe is safe to operate.



Thank you

Documents

Global Buckling Design for Flexible Flowlines · 2019. 2. 8. · Pipe-lay Pressurization Trench Backfill Depressurise Leak Test Operate Trench and Backfill The flexible is pressurised